In Third Generation Partnership Project (3GPP) Long Term Evolution (LTE), as in any communications system, a mobile terminal, or User Equipment (UE), may need to contact the network (via a base station, or an enhanced or evolved Node B (eNB)) without having a dedicated uplink resource from the UE to the base station. To handle this, a random access procedure is available where a UE that does not have a dedicated uplink resource may transmit a signal to the base station. The first message of the random access procedure, which in LTE is a random access preamble, is typically transmitted on a special resource reserved for random access. In LTE, this resource reserved for random access is referred to as a Physical Random Access Channel (PRACH). This PRACH can, for instance, be limited in time and/or frequency. FIG. 1 is a schematic diagram illustrating an example of a random access preamble transmission on a PRACH in LTE.
For random access, the UE performs a cell search procedure whereby the UE detects a cell by using Primary and Secondary Synchronization Signals (PSS/SSS). The UE blindly searches for a number of different sequences and the detected sequences give a Physical Cell Identifier (PCI) of a detected cell. After detecting the cell, the UE reads a Master Information Block (MIB) transmitted on a Physical Broadcast Channel (PBCH) occupying a known resource. The MIB gives the UE information about a System Frame Number (SFN) and how to detect further system information. More detailed system information is then provided in a number of System Information Blocks (SIBs). The first SIB (SIB1) contains a cell identity of the cell and scheduling information on how to decode the following SIBs.
Information about the resources available for PRACH transmission is provided to the UE as part of the broadcasted system information in a second SIB (SIB2) (or as part of dedicated Radio Resource Control (RRC) signaling in case of, e.g., handover). The resources available for PRACH transmission consist of available preamble sequences and a time/frequency resource. In each cell, there are 64 preamble sequences available. Two subsets of the 64 preamble sequences are defined, where the set of preamble sequences in each subset is signaled as part of the system information. The time/frequency resources are also associated to Random Access Radio Network Temporary Identifier (RA-RNTI) in the following fashion:RA-RNTI=1+t_id+10*f_id where t_id is an index of a first subframe of the specified PRACH (0≦t_id<10), and f_id is an index of the specified PRACH within that subframe, in ascending order of frequency domain (0≦f_id<6). The RA-RNTI associated with random access preamble transmission identifies the time and frequency resources used for transmission of the random access preamble.
According to 3GPP Technical Specification (TS) 36.211 version 12.1.0, the random access preambles are generated from one or several Zadoff-Chu sequences. The set of 64 preamble sequences in a cell is found by including the available cyclic shifts from each Zadoff-Chu sequence and adding more Zadoff-Chu sequences as needed. The number of cyclic shifts in a Zadoff-Chu sequence depends on the number of samples of the cyclic shift (Ncs) given by the zero correlation zone configuration and whether unrestricted or restricted sets of cyclic shifts are used. The sequences to use and the number of cyclic shifts to use per sequence are signaled in the system information.
When performing a (contention-based) random access attempt, the UE selects at random one sequence in one of the subsets. The set from which the sequence is selected may depend on various factors intended to communicate some information to the base station (e.g., LTE currently defines two subsets where the selected subset informs the base station about the amount the amount of data that the UE would like to transmit on the Uplink Shared Channel (UL-SCH) in the third random-access step). As long as no other UE is performing a random access attempt using the same sequence at the same time instant, no collisions will occur and the attempt will, with a high likelihood, be detected by the base station.
In LTE, the random access procedure can be used for a number of different reasons. Among these reasons are:                Initial access (for UEs in the RRC_IDLE state)        Incoming handover        Resynchronization of the uplink        Scheduling request (for a UE that is not allocated any other resource for contacting the base station)        Positioning        
The contention-based random access procedure used in LTE Release 10 is illustrated in FIG. 2. FIG. 2 is a diagram showing signaling over the air interface for the contention-based random access procedure in LTE. The UE starts the random access procedure by randomly selecting one of the random access preambles available for contention-based random access (step 100). The UE then transmits the selected random access preamble on the PRACH to the LTE Radio Access Network (RAN) for reception by an eNB (step 102).
The RAN (specifically an eNB) acknowledges any random access preamble that it detects by transmitting a random access response (MSG2) including an initial grant to be used on the uplink shared channel, a Temporary Cell Radio Access Network Temporary Identifier (TC-RNTI), and a Time Alignment (TA) update based on the timing offset of the preamble measured by the eNB on the PRACH (step 104). The MSG2 is also referred to herein as a Random Access Response (RAR) or RAR message. The MSG2 is transmitted in the downlink to the UE using the Physical Downlink Shared Channel (PDSCH), and the corresponding Physical Downlink Control Channel (PDCCH) message that schedules the PDSCH contains a Cyclic Redundancy Check (CRC) which is scrambled with the RA-RNTI.
When receiving the RAR, the UE uses the grant to transmit a message (MSG3) that in part is used to trigger the establishment of RRC and in part to uniquely identify the UE on the common channels of the cell (step 106). The TA command provided in the RAR is applied in the uplink transmission in MSG3.
In addition, the eNB can also change the resources blocks that are assigned for a MSG3 transmission by sending an uplink grant that has its CRC scrambled with the TC-RNTI which was included in MSG2 (step 108). In this case the PDCCH is used to transmit the Downlink Control Information (DCI) containing the uplink grant.
MSG4, which is then contention resolving, has its PDCCH CRC scrambled with the C-RNTI if the UE previously has a C-RNTI assigned (step 110). If the UE does not have a C-RNTI previously assigned, its PDCCH CRC is scrambled with the TC-RNTI obtained from MSG2. In the first case the UE included C-RNTI into MSG3 whereas in the latter case it included a core network identifier.
The procedure ends with the RAN solving any preamble contention that may have occurred for the case where multiple UEs transmitted the same preamble at the same time (step 112). This can occur since each UE randomly selects when to transmit and which preamble to use. If multiple UEs select the same preamble for the transmission on the Random Access Channel (RACH), there will be contention between these UEs that needs to be resolved through the contention resolution message (MSG4). The case when contention occurs is illustrated in FIG. 3, where two UEs transmit the same preamble (p5) at the same time. A third UE also transmits at the same RACH, but since it transmits with a different preamble (p1) there is no contention between this UE and the other two UEs.
The UE can also perform non-contention-based random access. A non-contention-based random access or contention free random access can, e.g., be initiated by the eNB to get the UE to achieve synchronization in the uplink. The eNB initiates a non-contention-based random access either by sending a PDCCH order or indicating it in an RRC message. The later of the two is used in case of handover to another cell.
The eNB can also order the UE through a PDCCH message to perform a contention-based random access. The procedure for the UE to perform contention free random access is illustrated below and in FIG. 4. Similar to the contention-based random access the MSG2 is transmitted in the downlink to the UE and its corresponding PDCCH message CRC is scrambled with the RA-RNTI (steps 200-204). The UE considers the contention resolution successfully completed after it has received MSG2 successfully (step 206). For the contention free random access as for the contention-based random access, the MSG2 contain a timing alignment value. This enables the eNB to set the initial/updated timing according to the UEs transmitted preamble.
FIG. 5a shows the detailed timing of the basic random access preamble. The preamble is prefixed with a Cyclic Prefix (CP) to enable simple frequency domain processing. Its length is in the order of Transmission Gap Period (TGP)+Time Domain Scrambler (TDS)=97.5+5 microseconds (μs)=102.5 μs, where TDS corresponds to the maximum delay spread and TGP corresponds to the maximum round trip time. The CP insures that the received signal is always circular (after removing the CP in the random access receiver) and thus can be processed by Fast Fourier Transforms (FFTs). Therefore, the “active” random access preamble duration is 1000 μs−2·TGP−TDS=800 μs. The random access subcarrier spacing is 1/800 μs=1250 Hertz (Hz).
FIGS. 5b through 5d show the extended preamble formats. Format 1 has an extended CP and is suited for cell radii up to approximately 100 kilometers (km). However, since no repetition occurs this format is only suited for environments with good propagation conditions. Format 2 contains a repeated main preamble and a CP of approximately 200 μs. With a random access opportunity length of 2 milliseconds (ms) the remaining guard period is also approximately 200 μs. This format supports cell radii of up to approximately 30 km. Format 3 also contains a repeated main preamble and an extended CP. Using a random access opportunity length of 3 ms this format supports cell radii of up to approximately 100 km. As opposed to format 1, format 3 contains a repeated preamble and is therefore better suited for environments with bad propagation conditions.
The requirements on the sequence comprising the preamble are two-fold: good Auto-Correlation Function (ACF) properties and good Cross-Correlation Function (CCF) properties. A sequence that has ideal (periodic) ACF and CCF properties is the Zadoff-Chu sequence. The periodic ACF of the Zadoff-Chu sequence is only non-zero at time lag zero (and periodic extensions) and the magnitude of the CCF is equal to the square root of the sequence length N. Due to special properties of Zadoff-Chu sequences the number of sequences is maximized if N is chosen prime. This together with the requirement that the effective random access bandwidth N·1250 Hz should fit into 1.05 Megahertz (MHz) leads to N=839.
A Zadoff-Chu sequence of length N can be expressed, in the frequency domain, as:
            X      ZC              (        u        )              ⁡          (      k      )        =      e                  -        j            ⁢                          ⁢      π      ⁢                          ⁢      u      ⁢                          ⁢                        k          ·                      (                          k              +              1                        )                          N            where u is the index of the Zadoff-Chu sequence within the set of Zadoff-Chu sequences of length N. Out of one Zadoff-Chu sequence—in the following also denoted root sequence—multiple preamble sequences can be derived by cyclic shifting. Due to the ideal ACF of the Zadoff-Chu sequence multiple mutually orthogonal sequences can be derived from a single root sequence by cyclic shifting one root sequence multiple times the maximum allowed round trip time plus delay spread in the time domain. The correlation of such a cyclic shifted sequence and the underlying root sequence has its peak no longer at zero but at the cyclic shift. If the received signal now has a valid round trip delay—i.e., not larger than the maximum assumed round trip time—the correlation peak occurs at the cyclic shift plus the round trip delay which is still in the correct correlation zone (see FIG. 6). For small cells with up to 1.5 km radii, all 64 preambles can be derived from a single root sequence and are therefore orthogonal to each other. In larger cells not all preambles can be derived from a single root sequence and multiple root sequences must be allocated to a cell. Preambles derived from different root sequences are not orthogonal to each other.
One disadvantage of Zadoff-Chu sequences is their behavior at high frequency offsets. A frequency offset creates an additional correlation peak in the time domain. A frequency offset has to be considered high if it becomes substantial relative to the random access sub-carrier spacing of 1250 Hz, e.g. from 400 Hz upwards. The offset of the second correlation peak relative to the main peak depends on the root sequence. An offset smaller than Target Cell Selection (TCS) may lead to wrong timing estimates, whereas values larger than TCS increase the false alarm rate. In order to cope with this problem LTE has a high speed mode (or better high frequency offset mode) which disables certain cyclic shift values and root sequences so that transmitted preamble and round trip time can uniquely be identified. Additionally, a special receiver combining both correlation peaks is required. For cells larger than approximately 35 km no set of 64 preambles exists that allows unique identification of transmitted preamble and estimation of propagation delay, i.e. cells larger than 35 km cannot be supported in high speed mode.
The random access preamble sequences are ordered according to a specified table. The table was designed by first separating all PRACH sequences into two groups based on the Quadrature Phase Shift Keying (QPSK) Cubic Metric (CM) value using a fixed 1.2 decibel (dB) threshold. The sequences with low CM are more suitable to assign to large cells than the sequences with high CM. Within each CM group (high and low) the sequences are further grouped according to the maximum allowed cyclic shift (Smax) at high speed.
In order to receive various types of DCI as well as to receive the RAR response message during random access, a UE performs PDCCH monitoring. In particular, a UE monitors a common search space and a UE specific search space in the PDCCH. In each search space, a limited number of candidates or equivalently PDCCH transmission hypothesis is checked, in every downlink subframe. These are known as blind decodes, and the UE checks whether any of the transmitted DCI messages is intended for it. PDCCH employs an interleaving technique where a coded DCI is spread out over resource elements covering a large frequency range.
The UE monitors the following Radio Network Temporary Identifiers (RNTIs) that are associated with the random access and paging procedures for each associated search spaces on PDCCH:                The RA-RNTI for MSG2 is monitored in the common search space.        The TC-RNTI for MSG3 is monitored in the common search space, for reallocating the MSG3 in frequency.        The TC-RNTI for MSG4 is monitored in the common search and UE specific TC-RNTI search space.        The C-RNTI for MSG4 is monitored in the common search and UE specific C-RNTI search space.        The Paging Radio Network Temporary Identifier (P-RNTI) is monitored is monitored in the common search space.        
Work in 3GPP is ongoing on reducing device complexity in order to address a potential Machine Type Communication (MTC) market requiring low device cost. One identified method for reducing cost is to reduce the UE received bandwidth. In other words, the received bandwidth of a MTC UE would be less than the system bandwidth of the downlink from the LTE RAN. The bandwidth of the MTC UE can be reduced by limiting the number of schedulable resource blocks for the physical data channels (e.g., PDSCH) while still receiving physical control channels (e.g., PDCCH) over the entire bandwidth or by reducing the bandwidth of all receptions.
If the bandwidths of some UEs are reduced, then the message transmissions, UE specific or common, that may address these UEs need to respect these bandwidth limitations. This means that a common transmission such as the RACH MSG2 transmission (for which a single transmission may contain random access responses to several different UEs) needs to be bandwidth limited, which may significantly reduce the feasible size of MSG2 and hence the number of PRACH preambles that can be acknowledged simultaneously in MSG2. This will reduce the RACH capacity significantly. In addition, for scenarios where UEs do not receive the entire system bandwidth even for physical control channels (e.g., PDCCH), these UEs will not be able to receive scheduling information for MSG2 and, therefore, will not know where to look for the MSG2.
As such, there is a need for systems and methods for random access for UEs, such as MTC UEs, having a reduced received bandwidth.